In Vivo Dynamic Deformation of Articular Cartilage in Intact Joints Loaded by Controlled Muscular Contractions

Abstract

When synovial joints are loaded, the articular cartilage and the cells residing in it deform. Cartilage deformation has been related to structural tissue damage, and cell deformation has been associated with cell signalling and corresponding anabolic and catabolic responses. Despite the acknowledged importance of cartilage and cell deformation, there are no dynamic data on these measures from joints of live animals using muscular load application. Research in this area has typically been done using confined and unconfined loading configurations and indentation testing. These loading conditions can be well controlled and allow for accurate measurements of cartilage and cell deformations, but they have little to do with the contact mechanics occurring in a joint where non-congruent cartilage surfaces with different material and functional properties are pressed against each other by muscular forces. The aim of this study was to measure in vivo, real time articular cartilage deformations for precisely controlled static and dynamic muscular loading conditions in the knees of mice. Fifty and 80% of the maximal knee extensor muscular force (equivalent to approximately 0.4N and 0.6N) produced average peak articular cartilage strains of 10.5±1.0% and 18.3±1.3% (Mean ± SD), respectively, during 8s contractions. A sequence of 15 repeat, isometric muscular contractions (0.5s on, 3.5s off) of 50% and 80% of maximal muscular force produced cartilage strains of 3.0±1.1% and 9.6±1.5% (Mean ± SD) on the femoral condyles of the mouse knee. Cartilage thickness recovery following mechanical compression was highly viscoelastic and took almost 50s following force removal in the static tests.

Fig 1

(a) Exposed mouse knee preparation showing the medial tibial plateau (T), and the medial femoral condyle (F) with the meniscus removed. (b) Tibio-femorol cartilage-on-cartilage space decreased to zero during muscular loading. Arrows show cartilage deformation area.

Fig 2. Normalized (relative to maximum = 100%) knee extensor forces as a function of time.

The medial tibio-femoral joint space narrowed during loading with (a) 35% of muscular loading (n = 7), the two cartilage surfaces never touch and no sign of cartilage deformation was noticed at this force. (b) 50% of muscular loading (n = 7), the cartilage-on-cartilage space reached zero and cartilage thickness decreased for these loading conditions. In both cases, medial tibio-femoral cartilage-on-cartilage space returned back to its original position in ~20s following the removal of force.

Fig 3. Normalized (relative to maximum = 100%) knee extensor forces as a function of time.

Muscles were stimulated for 8s at a voltage and frequency producing approximately (a) 50% of the maximal muscle force, cartilage compressive strain (mean ±1SD; n = 7) increased almost linearly 3s after the force application. Cartilage takes 20s for full recovery upon unloading. (b) 80% of the maximal isometric force. Cartilage compressive strain (mean ±1SD; n = 7) increased rapidly and reached near steady state conditions at 6s following force application. Cartilage tissue recovered to its original shape within approximately 45s following force removal.

Fig 4. Normalized (relative to maximum = 100%) knee extensor forces as a function of time.

Muscles were stimulated 15 times for 0.5s every 4s at a voltage and frequency producing approximately (a) 50% of the maximal muscle force. Cartilage compressive strain (mean ±1SD; n = 6) increased as a function of the number of muscular contractions. Full cartilage recovery takes around 20s upon unloading. (b) Loading using approximately 80% of the maximal isometric force. Cartilage strain (mean ±1SD; n = 6) increased with increasing the number of muscular contractions and reached its maximum at the last contraction. Cartilage tissue recovered to its original shape within approximately 35s following force removal.

Fig 5. Peak compressive strains as a function of normalised muscle force for static and dynamic loading.

For static loading, articular cartilage surfaces do not touch below approximately 40% of maximal muscle force, then cartilage strain increased between about 40–70% of the maximal force, and remained almost constant from 70–80% of maximal force (n = 7). For dynamic loading, articular surfaces did not touch below ~40% of the maximal isometric muscle force. Beyond ~40% force porduction, cartilage strains increased almost lineary between 50–80% of the maximal force (n = 6). Vertical bars on the side show significant differences between groups.